i. Coronary Artery Disease
Coronary artery disease continues to be one of the leading causes of morbidity and mortality, throughout the world. The typical etiology of coronary artery disease is characterized by the build-up of atherosclerotic plaque within the coronary arteries. Such deposits of atherosclerotic plaque tend to fully or partially block the flow of blood through the affected coronary arteries, and if untreated can result in myocardial ischemia, infarction and death.
For many years, the traditional surgical treatment of coronary artery disease has been coronary artery bypass surgery. In traditional coronary artery bypass surgery, the patient is generally anesthetized and placed on cardiopulmonary bypass. A thoracotomy is performed and the obstructed coronary blood vessels are exposed by surgical dissection. One or more segments of the patient's saphenous vein or internal mammary artery is/are harvested for use as bypass graft(s). The harvested segment(s) of vein or artery is/are then anastomosed to the obstructed coronary artery(ies) to form bypass conduit(s) around the arterial obstruction(s). Such traditional coronary artery bypass surgery is expensive, extremely invasive, and is associated with significant operative and perioperative complications.
One alternative to traditional coronary artery bypass surgery is balloon angioplasty. In balloon angioplasty, a flexible guide catheter is percutaneously inserted into a peripheral artery (e.g., the femoral artery) and is transluminally advanced through the vasculature until the distal tip of the catheter is within an obstructed coronary artery. Thereafter, a balloon catheter is passed through the guide catheter and into the obstructive lesion. The balloon of the balloon catheter is inflated one or more times to dilate coronary artery in the region of the obstructive lesion. These balloon angioplasty procedures tend to be less expensive and less traumatic than traditional coronary artery bypass surgery. However, balloon angioplasty procedures of this type have been associated with a significant incidence of restenosis at the angioplasty site. The cause and mechanism of such restenosis continues to be the subject of ongoing study. However, such restenosis has generally been attributed to either a) an increase in the mass of the artery wall (e.g., neointima formation), b) a thickening of the artery wall without substantial change in it's mass (e.g., vascular remodeling) and/or c) radial contraction of the balloon-dilated artery wall upon healing of cracks and fissures that have been created by the balloon dilation process.
Another alternative to traditional coronary artery bypass surgery is transluminal atherectomy or ablation of the obstructive matter within the coronary artery. These transluminal atherectomy or ablation procedures are performed by passing a catheter-mounted ablation apparatus through the vasculature to the site of the coronary obstruction the catheter-mounted ablative apparatus is then utilized to cut, shave, sonicate, pulverize or otherwise ablate the obstructive matter from the lumen of the coronary artery. These atherectomy or ablative procedures must be performed with caution to avoid abrasion or damage to the artery wall, as such abrasion or damage can result in excessive scaring and subsequent reclusion of the artery lumen. Furthermore, these atherectomy or ablative procedures may, in some cases at least, be confounded by the need to meticulously contain and remove the severed fragments of obstructive matter in order to prevent such fragments of obstructive matter from escaping into the patient's circulatory system. Examples of such atherectomy catheters and other catheter-mounted ablative apparatus are described in U.S. Pat. No. 3,433,226 (Boyd), U.S. Pat. No. 3,823,717 (Pohlman, et al.), U.S. Pat. No. 4,808,153 (Parisi), U.S. Pat. No. 4,936,281 (Stasz), U.S. Pat. No. 3,565,062 (Kuris), U.S. Pat. No. 4,924,863 (Sterzer), 4B70,953 (Don Michael, et al.), U.S. Pat. No. 5,069,664 (Suess, et al.), U.S. Pat. No. 4,920,954 (Alliger, et al.) and U.S. Pat. No. 5,100,423 (Fearnot), as well as foreign patents/patent publications EP0347098A2 (Shiber), WO87-05739 (Cooper), WO89-06515 (Bernstein, et al.), WO90-0130 (Sonic Needle Corp.), EP316789 (Don Michael, et al.), DE 3,821,836 (Schubert), DE2438648 (Pohlman), and EP 0443256A1 (Baruch).
Other alternatives to traditional coronary artery bypass surgery have included minimally invasive endoscopic procedures which, ostensibly at least, can be performed through small (e.g., 1–3 cm) incisions formed in the patient's chest wall, by insertion of a thoracoscope and associated operative instruments through such incisions. One such thoracoscopic coronary bypass procedure is described in U.S. Pat. No. 5,452,733 (Sterman et al.). If perfected, these minimally invasive coronary artery bypass procedures may lessen the discomfort and length of recovery time experienced by patients who undergo such minimally invasive procedures vis a vis those who undergo traditional coronary artery bypass surgery. However, the performance of endoscopic surgical procedures of this type typically requires a great deal of operator skill and training. Furthermore, as with traditional coronary artery bypass surgery, the patients on whom these thoracoscopic procedures are performed are likely to undergo general anesthesia (with or without cardiopulmonary bypass) and the creation of a pneumothorax due to the formation of full-thickness incision(s) in the chest wall. Thus, many of the drawbacks associated with traditional coronary artery bypass surgery, are also associated with these minimally invasive thoracoscopic procedures.
ii. Transmyocardial Revascularization
Another type of procedure which has been devised for improving blood flow to ischemic regions of the myocardium is known as transmyocardial revascularization (TMR). These TMR procedures generally involve the formation of tunnels or passageways through the myocardial muscle for the purpose of providing improved blood flow. In one such TMR procedure, a tissue-boring device, such as a laser, is utilized to form a series of small-diameter passageways from the epicardial surface of the heart, through the myocardium, and into the left ventricle. Jeevanandam, et al., Myocardial Revascularization By Laser-Induced Channels, Surgical Forum XLI, 225–227 (October 1990); also see, U.S. Pat. No. 4,658,817 (Hardy).
A variant of the above-described TMR procedure is described in U.S. Pat. No. 5,389,096, (Aita, et al.), wherein a catheter-mounted tissue-boring apparatus (e.g., a laser) is advanced into a chamber (i.e., left ventricle) of the heart and is used to form a plurality of blind, partial-thickness passageways from the chamber of the heart into the myocardium.
Modified TMR procedures have also been described wherein an internally valved transmyocardial passageway is formed between a coronary artery and the left ventricle of the heart, such that blood from left ventricle may flow into the coronary artery. These modified TMR procedures, hereinafter generally referred to as “Transmyocardial Direct Coronary Revascularization” (TMDCR), are described in U.S. Pat. No. 5,287,861 (Wilk), U.S. Pat. No. 5,409,019 (Wilk), and U.S. Pat. No. 5,429,114 (Wilk). At least some of these TMDCR methods require that a catheter be introduced into the obstructed coronary artery and advanced through the obstructive lesion. After the catheter has been advanced through the obstructive lesion, the distal tip of the catheter is stirred or bent toward the artery wall and a tissue-penetrating element is passed through the artery wall, through the adjacent myocardium, and into the chamber of the left ventricle. Also, in this previously described TMDCR method, a stent or valving apparatus is required to be positioned within the transmyocardial passageway to perform a one-way valving function (i.e., to open and close the transmyocardial passageway in accordance with changes in the systolic-diastolic cardiac cycle).
These TMDCR methods, previously described in U.S. Pat. No. 5,287,861 (Wilk), U.S. Pat. No. 5,409,019 (Wilk) and U.S. Pat. No. 5,429,114 (Wilk), may be difficult or impossible to perform in patients who suffer from total or near total obstructions of a coronary artery, because of the necessary for advancing the catheter through the coronary artery obstruction to accomplish creation of the transmyocardial passageway at a location which is downstream of the coronary obstruction. Furthermore, because these previously described TMDCR methods require placement of a stent within the transmyocardial passageway, such procedures are necessarily associated with procedural complexities associated with measuring and pre-cutting the stent to a precise length so that it fits within the transmyocardial passageway without protruding into the chamber of the left ventricle and/or the lumen of the coronary artery. Also, any stent which is positioned solely within the transmyocardial passageway may be subject to repetitive flexing and/or stressing as the myocardium undergoes its normal contraction and relaxation. Such repeated flexing and/or stressing of the intramyocardial stent may lead to unwanted migration, dislodgement or damage of the stent.
iii. Intermittent Coronary Sinus Occlusion For Coronary Retroperfusion
Yet another procedure which has been proposed as a means for treating acute myocardial ischemia is known as intermittent coronary sinus occlusion (ICSO). In many if not all ICSO procedures, the inflatable balloon is placed in the coronary sinus and is attached to a mechanical pump. The mechanical pump operates to intermittently inflate and deflate the balloon so as to intermittently occlude the coronary sinus. Such intermittent inflation and deflation of the balloon may be linked to the coronary sinus pressure so as to optimize the retroperfusion of the ischemic myocardium. Specific ICSO procedures have been described in Belamy, R. F: (et al.) Effect of Coronary Sinus Occlusion on Coronary Pressure-Flow Relations, Am. J. Physiol. 239 (Heart Circ. Physiol. 8) HS 57-HS64, 1980; Pantely, G. A. (et al.) Effect of Resistance, And Zero Flow Pressure During Maximum Vasodilation in Swine, Cardiovascular Research, 22:79–86, 1988.
Because the coronary sinus balloon is typically mounted on a percutaneously inserted catheter and is connected to an extracorporeally located pumping system, the clinical usefulness of these ICSO procedures is presently limited to temporary applications intended to minimize heart muscle damage following infarction or during acute periods of myocardial ischemia. However, the general concept of these ICSO procedures could be applicable for the long-term treatment of chronic myocardial ischemia if a totally indwelling system could be devised which would eliminate the need for continued coronary sinus catheterization and/or the deployment of an extracorporeal pumping apparatus connected to such catheter.
When considering the manner in which the ICSO procedures operate, it is helpful to bear in mind that the human circulatory system functions to send oxygenated blood from the aorta to the heart muscle (myocardium) via the left and right coronary arteries (small and large arteries). Following the oxygen/metabolite exchange in the myocardium, the venous system returns the de-oxygenated blood via the epicardial veins (large and small veins). A small fraction of the de-oxygenated blood returns through a set of small vessels which empty directly into the chambers of the heart called the Thebesian veins. Support exists in clinical literature that these Thebesian vessels can carry as much as 90 percent of the total coronary inflow back into the chambers of the heart. The Thebesian vessels connect to both the venous and arterial beds in an interface at the capillary level. Based upon this knowledge of heart function, researchers have suggested that occluding the venous system (e.g. blocking the coronary sinus or other large vein), does not compromise heart function and may in fact create a beneficial effect in cases of diseased or ischemic myocardium.
One particular type of ICSO procedure, known as pressure-controlled intermittent coronary sinus occlusion (PICSO), and apparatus for performing such procedure are described in European Patent Application No. EP0230996 to Mohl. In this PICSO procedure, an external balloon catheter is inflated in the coronary sinus for a period of time (several cardiac cycles). This PICSO procedure is typically performed in a hospital setting, during a coronary catheterization procedure. This PICSO procedure has been shown to elevate the pressure in the venous bed, providing a therapeutic benefit. (see, Mohl, The Development and Rationale of Pressure Controlled Intermittent Coronary Sinus Occlusion-A New Approach to Protect Ischemic Myocardium, Wiener klinische Wochenschrift, pp 20–25, Jan. 6, 1984; also see, Moser, Optimization of Pressure Controlled Intermittent Coronary Sinus Occlusion Intervals by Density Measurements, in The Coronary Sinus, Eds. Mohl, Wolner, Glogar, Darmstadt: Steinkopff Verlag, 1984 pp. 529–536; Schreiner, The Role of Intramyocardial Pressure During Coronary Sinus Interventions: A Computer Model Study, IEEE Transactions on Biomedical Engineering, Vol. 37, No. 10, October (1990). In addition, research has shown that, in patients who suffer from occlusive coronary artery disease, venous occlusion may actually prevent or reduce the size of a subsequent infarct (heart attack). (Kralios, Protective Effect of Coronary Sinus Obstruction from Primary Ischemia-Induced Ventricular Fibrillation in the Dog, Am. Heart J (1993) 125:987; Lazar, Reduction of Infarct Size With Coronary Venous Retroperfusion, Circulation (1992) 86:II 352).
There currently exists a need to provide minimally invasive methods and devices which have the capability of achieving therapeutic effects similar to those of the above-described PICSO procedure of the prior art, but which do not require continuing cardiac catheterization of the patient in order for the beneficial effects of the procedure to remain. Accordingly, it is the object of the present invention to describe various devices that can be implanted percutaneously in the patient's coronary sinus or great cardiac vein where they remain implanted to achieve partial or total occlusion of the coronary venous system. One mechanism of the therapeutic benefits seen clinically are understood to be based upon the slowing of the arterial flow and subsequently the increased the dwell time of the blood in the capillary bed allowing for an increased oxygen-metabolite exchange. The benefits of this phenomenon are better oxygen uptake and potentially a decrease in the infarct size in the event of a future cardiac arrest, and/or potentially global ischemic protection. Another mechanism providing potential benefit is the resultant redistribution of flow through collateral capillary beds, allowing perfusion of regions of the heart that may be otherwise ischemic (lack of perfusion of blood flow due to disease in vessels feeding those regions). In addition, angiogenesis, a stimulation of the endothelial cells lining the internal walls of vessels to form new blood vessels, may be stimulated as a compensatory mechanism in response to the disruption of normal venous blood flow, thereby providing additional myocardial perfusion through newly created blood flow conduits.
In view of the above-summarized shortcomings and complexities of the previously described TMDCR methods, there exists a need in the art for the development of improved TMDCR methods and associated apparatus which may be utilized without the need for cumbersome stenting of the transmyocardial passageway and/or implantation of one-way valving apparatus within the transmyocardial passageway. Also, there exists a need for the development of a new TMDCR methods which can be performed in patients who suffer from total or near total coronary artery occlusion(s), without the need for advancing a catheter through such coronary artery occlusion(s).